<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    ul {
      padding: 0;
    }

    li {
      list-style: none;
      text-align: center;
      border-radius: 5px;
      background: skyblue;
    }

    ul {
      display: flex;
      flex-wrap: wrap;
      width: 100%;
      height: 100%;
    }

    li {
      width: 30%;
      height: 300px;
      margin-right: 5%;
      margin-bottom: 5%;
    }

    li:nth-of-type(3n) {
      margin-right: 0;
    }

    li:nth-of-type(n+7) {
      margin-bottom: 0;
    }
  </style>
</head>

<body>
  <div class="box">
    <ul>
      <li>1</li>
      <li>2</li>
      <li>3</li>
      <li>4</li>
      <li>5</li>
      <li>6</li>
      <li>7</li>
      <li>8</li>
      <li>9</li>
    </ul>
  </div>
</body>

</html>